The invention relates to the general field of ventilating a turbine section of a turbomachine. The invention relates more particularly to a device for regulating the flow rate of air for feeding to a turbine ventilation cavity in a turbine section of an aviation turbomachine.
In the present description, the term “ventilating” is used in respect of a turbine section to cover both a function of cooling the various vanes and/or blades of the turbine section, and a purging function to prevent hot air in the primary flow passage through the turbine from being reintroduced elsewhere.
When an aviation turbomachine is operating at high speed (typically at full throttle), the temperatures of the gases flowing in the primary flow passage of the turbomachine reach values that cannot be withstood without damage by the parts situated outside said passage, in particular parts situated around the turbine section(s) of the turbomachine (low pressure turbine and/or high pressure turbine). It is thus essential to prevent the gas stream that is flowing in the primary flow passage from penetrating outside that passage when the turbomachine is operating at high speeds.
For this purpose, it is known to feed compressed air to a cavity (referred to herein as the turbine ventilation cavity) that is formed around the outer casing of the turbine and that communicates with the flow passage for the gas stream through the turbine section(s), the compressed air being fed via a circuit comprising a series of orifices and bushings. Since the air in the turbine ventilation cavity is taken from a compressor state of the turbomachine, the turbine ventilation cavity is at a pressure that is higher than that existing in the flow passage for the gas stream through the turbine. As a result, the compressed air penetrating into the flow passage via the air insertion orifices serves to keep the gas stream that is flowing through the turbine sections confined inside the flow passage. The turbine is then said to be properly purged. The compressed air present in the turbine ventilation cavity is also used for cooling certain parts of the turbine, such as for example the first nozzle of the low pressure turbine (for a turbomachine of two-spool architecture).
The air in the turbine ventilation cavity is generally conveyed by tubular ducts connecting the turbine ventilation cavity to a take-off cavity that communicates with the flow passage for the stream of air passing through the compressor of the turbomachine. Diaphragms may be located inside these ducts in order to calibrate the flow rate of the air that is taken off. A check valve may also be located inside each duct at its end leading into the turbine ventilation cavity. In the event of the duct accidentally breaking and the pressure inside it dropping, the check valve prevents the gas stream flowing in the primary passage of the turbine from escaping therefrom. Reference can be made to document EP 1 312 763 that describes an embodiment of a check valve.
Nevertheless, taking compressed air to deliver it to the turbine ventilation cavity via ducts provided with flow rate calibration diaphragms as described above, does not enable the flow rate of the air that is taken off to be regulated as a function of the operating speed of the turbomachine. Since the diaphragms disposed in the ducts are of constant flow section, the air take-off flow rate is substantially constant for the various operating speeds when expressed as a percentage of the flow rate of air flowing in the primary passage of the compressor. It is thus common practice for the diaphragms to be dimensioned so as to enable air to be taken off at a flow rate that is sufficient to ventilate turbines (i.e. purge them and cool parts thereof) when the turbine is operating at high speed (such as full throttle). In flight at a lower speed of operation, where temperature levels in the turbomachine are not so high (such as when cruising), a smaller air flow rate would be sufficient for ventilating the turbines, so as a result too much air is taken off during such speeds of operation. Unfortunately, the air that is taken from the compressor and reintroduced further downstream into the turbine at a location where the pressure level is lower does less work in the turbine than it would have done if it had not been taken off. This has the consequence of penalizing fuel consumption. In order to optimize fuel consumption, it is therefore desirable to minimize air take-off from the compressor at each of the operating points of the turbomachine.